pci: sandbox: emul: Fix the call to pci_bus_find_devfn()

With the newly added test cases for PCI configuration access, we get:

  => ut dm pci_busdev
  Test: dm_test_pci_busdev: pci.c
  test/dm/pci.c:49, dm_test_pci_busdev(): SANDBOX_PCI_VENDOR_ID == vendor:
  Expected 4660, got 65535
  Test: dm_test_pci_busdev: pci.c (flat tree)
  test/dm/pci.c:49, dm_test_pci_busdev(): SANDBOX_PCI_VENDOR_ID == vendor:
  Expected 4660, got 65535
  Failures: 2

The bug only shows up when bus number is not equal to zero. This is
caused by the plain find_devfn parameter is passed to function call
pci_bus_find_devfn(), inside which find_devfn is compared to devfn
in the device's pplat structure. However pplat->devfn does not carry
the bus number. Fix this by passing find_devfn with bus number masked.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>